A drag race is an acceleration contest from a standing start between two vehicles over a measured distance. The accepted standard for that distance is either a quarter-mile (1,320 feet) or an eighth-mile (660 feet). These contests are started by means of an electronic device commonly called a "Christmas Tree" because of its multicolored starting lights. On each side of the Tree are seven lights: two small amber lights at the top of the fixture, followed in descending order by three larger amber bulbs, a green bulb, and a red bulb.
When the racer rolls forward into the staging beams, the front tires are positioned exactly on the starting line and the stage bulbs are lit on the Tree, which indicates that the vehicle is ready to race. When both vehicles are fully staged, the starter will activate the Tree, and each driver will focus on the three large amber lights on his or her side of the Tree.
Depending on the type of racing, all three large amber lights will flash simultaneously, followed four- or five-tenths of a second later by the green light (called a Pro Tree), or the three bulbs will flash consecutively five-tenths of a second apart, followed five-tenths later by the green light (called a Sportsman, or full, Tree). The car may not leave the start line before the green light is illuminated, and the first car across the finish line (without breaking out, if applicable) is the winner.
The reaction time of the driver and the car are critical factors in winning the race. Differences in reaction time of only a few thousandths of a second can mean the difference between winning and losing, and consistency of reaction is critical.
Most drag race cars use extensively modified automotive automatic transmissions. One of the more common modifications is to include a transmission braking system or "transbrake". The transbrake typically replaces the factory modulator valve with a special solenoid-operated valve controlled by electronic circuitry. By simultaneously applying low and reverse, a transbrake locks the automatic transmission, holding the vehicle's position. When the transbrake activating button is released the reverse hydraulic circuit is dumped allowing the vehicle to launch.
One of the most common transmissions to be so modified is the General Motors TurboHydramatic model 400 (TH400), which uses a band/drum arrangement for reverse. The reverse servo of the TH400 is easily accessible through a reverse servo cover. Other commonly used transmissions which can be modified with transbrakes are the Powerglide and TH350, both of which use a piston/clutch arrangement.
A drawback of commonly used transbrake controls is that, in order to release the transbrake, the hydraulic pressure on the reverse applicator (piston or servo) must be dumped as closely to instantaneously as possible. Any delay in pressure reduction translates directly into delay in reaction time and inconsistency in starting. Unfortunately, because of the design of automatic transmissions, with their maze of serpentine passages for fluid, there is significant hydraulic resistance as the fluid flows out of the reverse applicator, making the release of the applicator erratic and slower than desired. While attempts have been made to make the release of the applicator faster, they often require modification or replacement of the transmission case, a very expensive and complicated proposition.
Tobler's "Transmission Brake", U.S. Pat. No. 4,883,151, is for a transmission brake in which the conventional shifter is replaced by a control valve having a "brake" position. The remote valve and shifter mechanism would still entail the problems of slow fluid release noted above, with the additional factor of having to shift the transmission out of "brake" and into "low". Most drag racing applications currently use an electronic switch to initiate the release of the transbrake to avoid this delay.
Reid, in "Automatic Transmission for Racing Vehicle", U.S. Pat. No. 5,622,088, and related patent "Air Bleed and Adjustable Reverse Piston Release Rate in an Automatic Transmission for a Racing Vehicle", attempts to address the pressure dumping problem by providing special passages and adjustable valves for releasing the reverse piston, especially in the embodiment shown in FIG. 6. Nonetheless, the fluid still must release through the same passage as it was applied, at least partially, and Reid's invention involves extensive modifications to the transmission case.